How do I verify if a conductive mat is getting "Earthed"

Earthing mats do look like silicone rubber, but are made of, or coated with, some flexible conductive material. Therefore appearance alone is insufficient to assume it is not earthed.

If you have one of the screwdriver type mains testers, it is easy to verify earthing:

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Grab the metal clip at the back of the tester firmly using the mat as a glove - without directly touching the clip with any part of the body - and insert the screwdriver head into the live point of any mains socket that is switched on. If the mat is earthed, the tester will light up, as the tester works by earthing itself through the clip, then through the human body or other earthing connection.

Of course, also check if the tester lights up at the socket in the first place, by touching the clip with your hand.